**Core Concept**
The patient's symptoms suggest a parasitic infection causing autoinfection, which is a common feature of Enterobius vermicularis (pinworm) infection. Enterobius vermicularis is a small, white, thread-like worm that infects the human intestine and causes perianal pruritus, excoriation of skin, and nocturnal enuresis.

**Why the Correct Answer is Right**
The NIH swab method is a diagnostic technique used to detect the presence of Enterobius vermicularis eggs in the perianal region. The patient's symptoms and medical history are consistent with a pinworm infection, and the NIH swab method is a reliable method for diagnosing this condition. The test involves gently scraping the perianal region with a moist swab, which is then examined under a microscope for the presence of eggs.

**Why Each Wrong Option is Incorrect**
**Option B:** The string test is a diagnostic method used to detect the presence of hookworms in the stool, not Enterobius vermicularis. It involves passing a piece of string through the stool to collect a sample for examination.
**Option C:** Hetrazan test is a diagnostic method used to detect the presence of filarial parasites in the blood, not Enterobius vermicularis. It involves administering a test dose of diethylcarbamazine (hetrazan) and observing for a microfilarial response.
**Option D:** Encysted larvae on muscle biopsy are typically associated with Trichinella spiralis infection, not Enterobius vermicularis.

**Clinical Pearl / High-Yield Fact**
In cases of suspected pinworm infection, it is essential to perform a proper hand hygiene after touching the patient's stool or perianal region to prevent autoinfection.

**✓ Correct Answer: A. NIH swab method**